[Eon Netz manages the transmission grid in Schleswig-Holstein and Lower Saxony, about a third of Germany, hosting 6,250 MW of Germany's 14,250 MW installed wind-generating capacity at the end of 2003 (7,050 MW of Germany's 16,394 MW at the end of 2004, 7,600 MW of Germany's 18,300 MW at the end of 2005). The total production in Eon's system was 8.5 TW-h (11.3 TW-h in 2004, 11.6 TW-h in 2005), representing an average feed of 969 MW (15.5% of capacity) (1,295 MW, 18.3% of capacity in 2004; 1,327 MW, 18% of capacity in 2005). Germany's wind production as a whole was 14.8% (18.1% in 2004) of capacity and equal to less than 4.7% of demand (4.7% in 2005).]

For technical reasons, the intensive use of wind power in Germany is associated with significant operational challenges:

Only limited wind power is available. In order to cover electricity demands, traditional power station capacities must be maintained as so-called "shadow power stations" at a total level of more than 80% of the installed wind energy capacity, so that the electricity consumption is also covered during economically difficult periods.

Only limited forecasting is possible for wind power infeed. If the wind power forecast differs from the actual infeed, the transmission system operator must cover the difference by utilising reserve capacity. This requires reserve capacities amounting to 50-60% of the installed wind power capacity.

Wind power requires a corresponding grid infrastructure. The windy coastal regions of Schleswig-Holstein and Lower Saxony are precisely the places where the grids have now reached their capacity limits through wind power. At present, just under 300 km of new high-voltage and extra-high voltage lines are being planned there in order to create the transmission capacities required for transporting the wind power.

Electricity generation from wind fluctuates greatly

The level of wind power infeed fluctuates greatly depending on the prevailing wind strength. Due to these significant fluctuations, in 2003 the contribution made by wind power production to cover the respective peak load in the E.ON territory varied between zero in real terms and just under one third of the grid load [same in 2004].

Looked at over the course of the year, the availability of the installed wind power plants was relatively low:

Simultaneous wind power infeed was maximum 4,980 MW, equivalent to just under 80% of the
installed capacity [2004: 6,024 MW, 85% of capacity].

The average fed-in capacity was less than one sixth [15.5%] of the wind power capacity installed in the yearly average.

Over half the year, the wind power fed in was less than 11% [14% in 2004] of the wind power capacity installed in the yearly average. [For two thirds of the year, production was lower than the yearly average of 15.5%, 18.3% in 2004.]

The contribution of wind power to covering the daily grid peak load in 2003 varied between 0.1% and 32%.

Figure 4 shows an example of the wind power infeed pattern in the E.ON territory during a week with strong winds. The difference between minimum and maximum infeed in this example was over 4,300 MW -- equivalent to the capacity of six to eight large coal-fired power station blocks.

The wind power infeed changes can occur in a relatively short time. This can be seen in Figure 5, which shows the wind power infeed pattern in the E.ON control area in the week of 17th to 23rd November 2003. It is clear that on 19th November, the wind power infeed dropped very sharply -- by 3,640 MW within six hours, with an average value of 10 MW per minute.

[From 2005 report: "Whilst wind power feed-in at 9.15 am on Christmas Eve reached its maximum for the year at 6,024 MW, it fell to below 2,000 MW within only 10 hours, a difference of over 4,000 MW. This corresponds to the capacity of 8 × 500-MW coal fired power station blocks. On Boxing Day, wind power feed-in in the E.ON grid fell to below 40 MW."]

The weather determines the wind level

Both cold wintry periods and periods of summer heat are attributable to stable high-pressure weather systems. Low wind levels are meteorologically symptomatic of such high-pressure weather systems. This means that in these periods, the contribution made by wind energy plants to covering electricity consumption is correspondingly low.

The experience of the past year has shown that whenever electricity consumption was comparatively high because of the weather, namely during cold wintry or hot summer periods, wind power plants could make only a minor contribution towards covering consumption.

This relationship was again confirmed in Germany during the heatwave of July/August 2003 (Figure 6). The summer electricity consumption was at that time at an above-average high level due to the temperature. At the same time, traditional power stations had to partly reduce their capacity so as not to impermissibly heat up the rivers that serve as sources of cooling water. During this phase, wind power production was also very low due to the lack of wind and was not able to contribute towards relieving the strained supply situation.

Figure 7 shows that in the winter of 2003 also, the contribution of wind power towards covering load was low precisely in phases of particularly high electricity demand. The wind power infeed curve during the week of so-called "midweek peak load" in the E.ON grid in 2003 is shown.

In order to also guarantee reliable electricity supplies when wind power plants produce little or no electricity -- for example during periods of calm or storm-related shutdowns -- traditional power station capacities must be available as a reserve. The characteristics of wind make it necessary for these "shadow power stations" to be available to an extent sufficient to cover over 80% of the installed wind energy capacity. This means that due to their limited availability, wind power plants cannot replace the usual power station capacities to a significant degree, but can basically only save on fuel.

["In 2004 two major German studies investigated the size of contribution that wind farms make towards guaranteed capacity. Both studies separately came to virtually identical conclusions, that wind energy currently contributes to the secure production capacity of the system, by providing 8% of its installed capacity.

["As wind power capacity rises, the lower availability of the wind farms determines the reliability of the system as a whole to an ever increasing extent. Consequently the greater reliability of traditional power stations becomes increasingly eclipsed. As a result, the relative contribution of wind power to the guaranteed capacity of our supply system up to the year 2020 will fall continuously to around 4%.

["In concrete terms, this means that in 2020, with a forecast wind power capacity of over 48,000 MW (Source: dena grid study), 2,000 MW of traditional power production can be replaced by these wind farms."]

Only limited forecasting possible for wind power infeed -- demand for wind-based reserve capacity increases with new wind power construction

Large quantities of electrical energy cannot be directly stored. This means that every second, exactly the amount of energy must be fed into the grid that is taken out at the same time. If the amount fed in differs from the amount tapped, this can cause faults or even failure of the supply -- as confirmed in 2003 by the wide-scale power failures
in the USA, Italy, Sweden and Denmark.

The transmission system operators must therefore at all times ensure a balance in their control areas between generation and tapping (power-frequency control).

Generation in traditional power stations can be easily controlled in line with demand. As a result, in the past it was mainly only the time pattern of tapping from the grid that was relevant to power-frequency control. Thanks to constant consumption behaviour, this tapping can be forecast with a high degree of accuracy.

The increased use of wind power in Germany, however, has resulted in uncontrollable fluctuations now also occurring on the generation side due to the stochastic character of wind power infeed, thereby increasing the demands placed on control and bringing about rising grid costs.

So that stable grid operation is possible despite the high volatility of wind power infeed, transmission system operators depend on the most accurate possible forecasts of the expected wind power infeed.

To forecast wind power, E.ON Netz uses a complex forecasting system developed by ISET and based on the forecasting data of the German Meteorological Service.

The quality of wind power forecasting is to a great extent limited by the quality of the wind forecasting. Like all weather forecasting, this is only partly reliable.

In 2003, the average negative forecasting error for the E.ON control area was -370 MW, and the average positive forecasting error was 477 MW.

However, during individual hours the deviations reached much higher levels of up to ±2,900 MW [–2,532 MW, +3,999 MW in 2004]. This was equivalent to just under half the installed wind power capacity.

The transmission system operator must balance out differences between the wind power forecast and the wind power actually fed in by using the controlling power range and reserve capacity.

Of crucial importance to the wind-related demand for reserve capacity is the expected maximum forecast deviation and not, for example, the mean forecast error. This is because even if the actual infeed deviates from the forecast level only on a few days in the year, the transmission system operator must also be prepared for this eventuality and have sufficient capacity available so that a reliable supply is still guaranteed.

The massive increase in the construction of new wind power plants in recent years has greatly increased the need for wind-related reserve capacity in Germany. In 2003, costs amounting to around 100 million € for this were incurred in the case of E.ON Netz alone.

Operational experience over the past few years has shown that reserve capacities in the order of magnitude of up to 60% of the installed wind power capacity must be kept for wind balancing in years when wind levels are normal. The need for reserve capacity and the resulting costs will therefore continue to rise in future parallel to the further expansion of wind power.

One decisive factor for the further expansion of wind energy use will be the capacities of the electricity grids. Today, the grids in some regions of Germany, for example in Schleswig-Holstein and Lower Saxony, are already approaching their capacity limits. When the wind is strong, they are unable to take any additional wind power.

The reason: Up to now, electricity supplies in Germany have largely been decentralized, with power stations having been built across the country as close to the points of consumption as possible. This has made it possible to avoid transporting electricity across long distances.

The power grids were built to bring the energy from these power stations to the consumers, which has meant that, expressed in simple terms, energy has always flown in one direction and only across relatively short distances. This has changed with the boom in wind energy. An increasing number of wind parks have been and are being built primarily in coastal and relatively sparsely populated areas of low consumption, which in periods of strong wind generate more energy than the area in question consumes at the same time. Consequently, this surplus energy must be transported over long distances. The line grids in the coastal regions can no longer do this in their current state without limits.

To remedy wind-related congestions, E.ON Netz is planning around 110 km of new 110 kV high-voltage lines in Schleswig-Holstein. Approximately 180 km of high-voltage and
extra-high-voltage lines are being planned in Lower Saxony, including for the first time a new wind-related extra-high-voltage route in the Oldenburg Mčnsterland.

In both cases, the plans are based on pure onshore expansion scenarios. If offshore wind parks will also be built on a greater scale in the future, additional grid expansion measures would also be necessary in the extra-high-voltage grid.

Wind power does not only cause regional grid congestion in the north German Federal States. In Schleswig-Holstein and Lower Saxony, far more wind power is generated under conditions of high wind and low load than is consumed in these states. Since in the coming years the expansion of wind power is set to progress on the basis of political will, by the end of the decade at the latest, Schleswig-Holstein and Lower Saxony will be wind power export states across long distances. The same will probably also then apply to Mecklenburg-Vorpommern if the current offshore plans become reality. This will drastically change the current principle of decentralized electricity generation close to the point of consumption. Cross-border electricity trading will also be significantly hindered by increased grid congestion. New transport lines will be necessary on a large scale in order to bring wind power generated on the coast and at sea to the consumer centres in the Ruhr or Rhine-Main region. In its expert assessment relating to this, the Institute for Electrical Plant and Energy Management of the RWTH Aachen assumes that by 2016, up to 1,500 km of new high-voltage and extra-high-voltage power lines will be required for this in Germany (Energiewirtschaftliche Tagesfragen 9/2003, page 566).

In Schleswig-Holstein, due to the many wind power plants installed there, the grid capacities are now exhausted when there is strong wind. Although the approvals procedures for the required grid expansion measures have already been initiated, it can be assumed that it will be several years before the planned power lines are realized. So that additional wind parks can still be brought on line until completion of the grid expansion, in 2003 E.ON Netz GmbH introduced what is referred to as "generation management" in Schleswig-Holstein.

This refers to a temporary reduction in the power fed in by wind energy plants when there is strong wind in order to protect grid infrastructure such as overhead lines or transformers against supply-related overloads and to avoid supply failures.

Until the grid expansion is completed, new wind parks in Schleswig-Holstein can be granted only conditional grid connection approval. A condition is agreement to participate in the generation management.

Without generation management, further expansion of wind power in Schleswig-Holstein is for the time being not possible.

In view of the wind power-related grid congestions in Lower Saxony, E.ON Netz is also introducing generation management there.

The foreseeable further expansion of wind energy in Germany and Europe means that in future, it will be necessary to pay more attention than before to supply reliability when designing new wind energy plants.

The operational behaviour of wind power plants has so far differed greatly from that of traditional large power stations. Due to the massive and ongoing new expansion of wind power, it has therefore become increasingly difficult to guarantee the stability of the electricity supply -- particularly in the event of a power failure.

This means that wind power plants do not contribute to the same extent towards stabilising the grid frequency and to voltage stabilising as is the case with traditional power stations, which are actively involved in grid control.

But even more serious is the fact that wind power plants of the usual type have so far disconnected themselves from the grid even in the event of minor, brief voltage dips, whereas large thermal power stations are disconnected only following serious grid failures.

Faults in the extra-high-voltage grid can therefore result in all wind power plants in the affected region failing suddenly. This means that within a very short time, the wind power supply of up to 3,000 MW can fail, thereby putting the grid stability at risk.